1. Field of the Invention
The present invention relates to driving control of a fuel cell system which generates electricity by electrochemical reaction of hydrogen with oxygen.
2. Description of the Related Art
In recent years, fuel cells which use electrochemical reactions of hydrogen with oxygen to generate electricity have been a focus of attention as energy sources. For example, one type of fuel cell, a solid polymer electrolyte fuel cell, has a structure which is provided with a stack of plurally layered cells which are formed by sandwiching a solid polymer electrolyte layer from both sides with a hydrogen electrode (which may hereafter be referred to as an anode) and an oxygen electrode (which may hereafter be referred to as a cathode).
In the fuel cell, hydrogen gas is supplied to the anode, to act as fuel, and air is supplied to the cathode, to act as an oxidizing agent. The hydrogen supplied to the anode of the fuel cell reacts with a catalyst of the anode, thus generating hydrogen ions. These hydrogen ions pass through the solid polymer electrolyte layer, initiate an electrochemical reaction with the oxygen at the cathode, and generate electricity.
Supply amounts of hydrogen gas and air that are required for the electrochemical reaction in the fuel cell (which may hereafter be referred to as “reaction gases”) vary depending on a load that is applied to the fuel cell. Accordingly, a fuel cell system has been proposed in which, by controlling degrees of openness of pressure regulation valves (pressure regulators) or the like in accordance with the size of a load, flow distributions of each gas are made uniform and electricity-generating effectiveness of the fuel cell is raised (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 7-302603).
However, during operation of a fuel cell, the electrochemical reaction of the hydrogen with the oxygen is accompanied by the creation of water. The created water is used for cooling in the fuel cell system and, in order to prevent ingression thereof into a pressure regulator-driving section or the like, an exhaust device is operated and the water is exhausted to outside the fuel cell (see, for example, JP-A No. 2002-305017).
If the fuel cell system is below freezing point or the like or driving is stopped for a certain period, there is a risk that water that remains in the system may freeze at locations such as valves, pumps and the like. In particular, in a fuel cell system which drives a pressure regulator as described above, when the system is controlled in accordance with the size of a load at a very low temperature, below freezing point or the like, there is a risk that the pressure regulator may be frozen in a shut state, that the valve may bite into (get caught in) the ice when the valve is closed, or the like, and a valve mechanism of the pressure regulator may become incapable of proper driving and/or damaged.
Furthermore, even in a case in which water is exhausted to outside the system by an exhaust device as described above, it is not possible to prevent adherence of water at the pressure regulator itself. Therefore, the same problems may occur.